Fix encoder

What's Changed

• feat: add mux writer by @damianopetrungaro in #23
• ci: add codecov by @damianopetrungaro in #24
• test: cover exported functions by @damianopetrungaro in #25
• fix: encoders handle errors as strings by @damianopetrungaro in #26

Full Changelog: v0.2.2...v0.3.0